Parasites are degenerate forms of free-living or mutualistic organisms.
They became parasites when something went wrong as a result of the Fall.
For example, the parasite came to invade the wrong host or the wrong organ
within the host, or it changed to harm the host where it did not before.
Parasites are far from degenerate. They have lost features that are
familiar to us as nonparasites, but they also have acquired many other
highly sophisticated features and abilities, allowing them to find
their hosts, to survive their hosts' immune systems (often multiple hosts
for one parasite), and to survive some otherwise hostile environments
within their hosts. Creationists themselves tout the complexity of the
immune system; does not circumventing an
immune system
deserve at least as much credit? Fast-evolving viruses like the common
cold show that such adaptations are evolving all the time.
Here are just a few features that parasites have. Similar adaptations
are common (Hajek and St. Leger 1994; Zimmer 2000):
Sacculina, a parasitic barnacle, infests crabs. It prevents the
crab from molting and reproducing and induces the crab to care for
the parasite's brood as if it were the crab's. Even male crabs are
feminized to groom as if they had a female's brood pouch (Zimmer
2000, 79-82).
The larva of the Hymenoepimecis wasp parasitizes an orb-weaving
spider. When the larva is ready to pupate, it modifies the spider's
behavior to make it spin a cocoon for the wasp (Eberhard 2000).
The fungus Entomophthora muscae infects and kills house flies, but
before it kills them, it manipulates the fly's behavior to make it
crawl to a high place and adopt a sexually receptive pose, behaviors
that increase the likelihood of the fungus spreading to other flies
(Moller 1993).
Evolution often goes the other way; parasites that initially are very
harmful become more benign to their host over time. The virulence of a
pathogen is generally predictable on the basis of evolutionary
principles. For example, parasites are less virulent at low host
population densities where the parasites risk destroying available
hosts and themselves with them (Nesse and Williams 1994, 57-61; Zimmer
2000, 151-155).
Why do organisms have defenses against pathogens in the first place?
They would not have been needed in a pre-Fall world without pathogens,
and their complexity and effectiveness show that features such as
immune systems are not degenerate forms themselves.
References:
Eberhard, William G., 2000. Spider manipulation by a wasp larva.
Nature 406: 255-256.
Hajek, A. E. and R. J. St. Leger, 1994. Interactions between fungal
pathogens and insect hosts. Annual Review of Entomology 39:
293-322.
Moller, Anders Pape, 1993. A fungus infecting domestic flies
manipulates sexual behaviour of its host. Behavioral Ecology and
Sociobiology 33: 403-407.
Nesse, R. M. and C. G. Williams, 1994. (see below)
Zimmer, Carl, 2000. Parasite Rex. (see below)
Further Reading:
Nesse, Randolph M. and George C. Williams, 1994. Why We Get Sick, New
York: Times Books.
Sapolsky, Robert, 2003. Bugs in the brain. Scientific American
288(3)
(Mar.): 94-97.
Zimmer, Carl, 2000. Parasite Rex: inside the bizarre world of nature's
most dangerous creatures. New York: The Free Press.
